In at least one known CT system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the "imaging plane". The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a "view". A "scan" of the object comprises a set of views made at different gantry angles during one revolution of the x-ray source and detector.
In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts that attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
To reduce the total scan time, a "helical" scan may be performed. To perform a "helical" scan, the patient is moved while the data for the prescribed number of slices is acquired. Such a system generates a single helix from a one fan beam helical scan. The helix mapped out by the fan beam yields projection data from which images in each prescribed slice may be reconstructed.
In helical scanning, and as explained above, only one view of data is collected at each slice location. To reconstruct an image of a slice, the other view data for the slice is generated based on the data collected for other views. Helical reconstruction algorithms are known, and described, for example, in C. Crawford and K. King, "Computed Tomography Scanning with Simultaneous Patient Translation," Med. Phys. 17(6), Nov/Dec 1990.
In known CT systems, the x-ray beam from the x-ray source is projected through a pre-patient collimating device, or collimator, that defines the x-ray beam profile in the patient axis, or z-axis. The collimator typically includes x-ray absorbing material with an aperture therein for restricting the x-ray beam. Known apertures are typically linear, or rectangular, and the aperture width controls the slice thickness as measured along the z-axis. For example, by passing an x-ray beam through a collimator with a 10 mm aperture, i.e., a 10 mm collimator, the beam output from the collimator will have a 10 mm slice thickness.
Helical scans, as is known, typically are performed at a X:Y helical pitch, wherein helical pitch is the ratio of patient movement along the z-axis during one rotation of the x-ray source, X, to the slice thickness, Y, defined by the source collimator. For example, for a 1:1 helical pitch scan with a 5 mm collimator, the patient moves at a speed of approximately 5 mm/sec. Similarly, for a 1:1 helical pitch scan with a 10 mm collimator, the patient moves at a speed of approximately 10 mm/sec.
Slice thickness directly affects image resolution and scan efficiency. Particularly, smaller slice thicknesses typically provide a more detailed image resolution than larger slice thicknesses. However, larger slice thicknesses are more efficient than small slice thicknesses since more of the region is scanned with a large slice thickness in a shorter period of time.
Slice thickness, as is known, is related to both helical pitch and collimator size. Particularly, by reducing helical pitch, patient movement during x-ray source rotation is reduced, thus reducing the effective slice thickness, i.e., less of the patient is scanned during one gantry rotation. Slice thickness similarly is decreased by reducing collimator size. Alternatively, by increasing helical pitch, patient movement is increased thus increasing the effective slice thickness. Increasing the collimator size similarly increases the slice thickness.
Typically, an operator selects a slice thickness prior to a scan to optimize scan efficiency and image quality. Particularly, smaller slice thicknesses are preferable when scanning regions with multiple bony structures, i.e., when scanning the pancreas region. Larger slice thicknesses, however, are preferable when scanning regions with few bony structures, i.e., when scanning the liver region. Accordingly, and for example, an operator may choose a 10 mm collimator at a 1:1 helical pitch to scan a liver region, and the operator may choose a 5 mm collimator at a 1:1 helical pitch to scan a pancreas region.
Scans, however, often are performed for a region that includes different and adjacent sub-regions, i.e., a bony sub-region adjacent a non-bony sub-region. To optimize image quality and scan efficiency for such regions, an operator typically must use a "compromise" collimator size. Accordingly, the bony sub-regions are scanned with an overly broad slice thickness and the non-bony regions are scanned with an overly thin slice thickness. Scans with such "compromise" collimation, accordingly, are neither efficient nor practical for optimizing system performance and image quality.
Known methods of improving image quality and scan efficiency when scanning adjacent bony and non-bony regions typically include altering slice thicknesses during a scan. Particularly, different x-ray source collimators or different helical pitches are selected when scanning the different regions. For example, a 10 mm collimator may be used when scanning a region with few bony structures and a 3 mm collimator may be used when scanning a region with many bony structures. However, such methods require interrupting the scan before changing the helical pitch of the collimator. Until now, it was believed that failure to interrupt the scan when changing the slice thickness would cause a loss of data and a degradation in image quality. Accordingly, the known methods are both time consuming and inefficient.
It would be desirable to modify slice thickness during a scan without interrupting the scan. It also would be desirable to modify slice thickness without significantly increasing the processing time and without significantly decreasing image quality.